Helmet-Mounted Displays: - USAARL - The - U.S. Army

Optical Designs 71
Figure 3.4. Comparisons between refractive and catadioptric HMDs with
and without prism combiners.
distortions and aberrations increase rapidly (Buchroeder, 1987). An
example of a modest off-axis catadioptric design with a plano combiner is
shown in Figure 3.5 (Droessler and Rotier, 1989; Rotier, 1989). This
catadioptric design achieves a 50º x 60º FOV with a 10-exit pupil and 30mm
eye relief (measured from plano combiner intercept to apex of eye
along primary line of sight). However, note the optical complexity with 11
refractive elements and 3 reflective surfaces with very complex coatings for
both eyepiece reflective surfaces to maximize see-through and display
transmissions. The modest trapezoidal distortion of 7.5% (Figure 3.6) will
be aligned with the power combiner. Another promising HMD is the
Monolithic Afocal Relay Combiner (MONARC), which is an off-axis,
rotationally symmetrical lens system with modest FOV potential, but
excellent see-through approach (Figure 3.7). However, for any of the offaxis
binocular systems, the distortions will have to be corrected to achieve
point for point image alignment throughout the FOV.
The primary advantage of the off-axis reflective HMD design is that it
provides the highest potential percent luminance transfer from the display
with the most see-through vision and increased eye clearances for a given
FOV. The primary disadvantages are very complex optical designs, shape
distortions, and low structural integrity and stability of the reflective
surface. Figure 3.8 shows the conceptual drawings (top and side view) of
an off-axis HMD using the visor as the eyepiece. Note the locations of the
aerial images, which are shown for the left eye. The location of the relay
optics will be either on top of the helmet, or below, where both locations
have undesirable characteristics such as a high center of mass, or produce
lower obstructions to unaided vision. Also, note that the head seems to get
in the way with the optics or relay image. Again, there have been
prototypes and a few HUDs, but no production off-axis reflective HMDs.
Where there are no provisions for electronic distortion correction, as found
with NVGs, the off-axis designs become unacceptable from the keystone
or trapezoidal type distortions.
Pupil and nonpupil forming
A nonpupil forming virtual display uses a simple eyepiece to collimate
or focus a real image source. An example is NVG where eyepieces focus
the 18-mm phosphor screens to produce a 40º FOV. The display size,